Whey, the supernatant fluid derived from removal of some or all of the casein from milk, is a rich source of lactose and protein. Whey may be produced by acidification of skim milk to a pH of about 4.7, which causes the casein to precipitate. Casein can be further purified and used in cheese making, in manufacture of some plastics and for other purposes. Acidification of milk to produce whey may be performed by addition of lactic or other acid, producing "acid" whey, or enzymatically, producing "sweet" whey. Acid whey is produced, for example, as a byproduct of the process for making cottage cheese. Sweet whey is a byproduct of some other cheese making processes e.g. cheddar. Sweet and acid whey differ mainly in acidity (attributable to the presence of lactic or other acid), mineral content, and fat content. Sweet whey has a pH of about 5.9 to 6.5, containing about 0.5 weight % inorganic salts (also referred to as "ash") and about 0.2 to 0.4 weight % fat. Acid whey has a pH of about 4.3 to 4.6, containing about 0.7 to 0.8 weight % ash and about 0.05 to 0.1 weight % fat.
After the salts (i.e. ash) and lactic acid are reduced or substantially removed, whey can be used as an additive to animal feed or to a variety of human foods such as protein and citrus drinks, dry mixes, confectionery coatings, ice cream, bakery goods, and the like. A particular utility of whey, in the form of reduced mineral whey (RMW), is as an additive to human infant formula. By virtue of the removal of casein, whey produced from bovine milk has a protein composition which conforms closely to that of human milk, in contrast to the protein composition of whole bovine milk. For this reason, RMW derived from bovine milk is a particularly suitable infant formula additive. Sweet whey derived from cheese making processes is preferably used to produce RMW for infant formula, resulting in an efficient and profitable use of a by-product.
Electrodialysis (ED) is commonly employed to produce RMW for use in infant formula because of its gentle method of desalting. Typically, 85 to 95% of the minerals may be removed from whey in a batch or a continuous ED process. Worldwide ED production of demineralized whey is in excess of 150,000 metric tons (330 million pounds) of RMW solids (dry basis) per year. ED facilities may have an installed capacity to demineralize as much as 500,000 kg (1.1 million pounds) or more per day of fluid whey. ED methods which are particularly suitable for demineralization of whey are disclosed, for example, in commonly assigned U.S. Pat. No. 5,223,107, which is incorporated herein by reference.
In general, ED methods and apparatus purify through electric field-mediated transfer of ions through membranes from less concentrated compartments (diluting or permeate streams) to more concentrated compartments (concentrating or brine streams). Anion transfer and cation transfer membranes are alternated in ED methods and apparatus, the membranes being placed between an anode and a cathode across which an electric field is applied. Anion transfer membranes allow passage substantially only of negatively charged low molecular weight species (anions), and cation transfer membranes allow passage substantially only of positively charged low molecular weight species (cations). The combination of an anode, a cathode, and the alternating anion and cation transfer membranes therebetween is commonly referred to as an ED "stack".
Several problems are inherent to electrodialysis of whey. ED membranes are particularly vulnerable to fouling during purification of whey. Calcium is present in whey at relatively high concentrations, and during ED calcium salts can precipitate in the brine stream and on membrane surfaces. Acid may be added to the brine stream to prevent such precipitation: however, use of large volumes of acid creates cost and disposal problems.
Certain cation exchange membranes have proven particularly suitable for use in ED of whey. For example, cation exchange membranes based on sulfonated polystyrene are routinely employed for ED of whey, since they are particularly stable in the presence of alkaline and acid washing solutions used for sanitation of ED equipment.
Sulfonated polystyrene based cation exchange membranes were initially manufactured by a multi-step process: in the first step, monomers such as divinyl benzene and styrene, in a water insoluble organic solvent such as diethyl benzene, were polymerized on a reinforcing fabric. In the second step, the resulting solid polymer was sulfonated in a second water insoluble organic solvent, such as ethylene dichloride. The finished membrane was produced by washing with a polar organic solvent such as methanol and then neutralizing with aqueous sodium bicarbonate. The multi-step process causes significant chemical disposal problems, since the monomers employed are all water insoluble, and the polymerization and subsequent reactions are therefore carried out in water insoluble solvents. Another disadvantage of the multi-step process is that sulfonation may occur more heavily at the surface of the membrane than in its interior, producing membranes having high electrical resistance.
U.S. Pat. No. 4,540,762 discloses copolymerization of sodium-N-(4-sulfophenyl) maleimide and a styrene sulfonate salt to produce a linear (i.e., not cross-linked), water soluble polyelectrolyte for use as a deflocculating agent in water-based drilling muds. U.S. Pat. No. 4,511,712 discloses a method of isolating ionic polymers, including styrene homopolymers, in the salt form. U.S. Pat. No. 4,060,673 discloses salts of polystyrene sulfonates to produce water soluble ion exchange membranes for use as permselective barriers in organic electrode batteries. U.S. Pat. No. 4,110,366 discloses a process for producing an alkali metal styrene sulfonate by an extraction/back-extraction process.
The ion exchange capacity obtainable from quaternary ammonium styrene sulfonate salts as disclosed in commonly assigned U.S. Pat. No. 5,203,982 may be limited by the solubilities of such styrene sulfonate quaternary ammonium salts in polar solvents. Such patent is also incorporated herein by reference. In addition, since styrene sulfonate quaternary ammonium salts are relatively large molecules, the resulting membranes have sufficiently large interstices that lactose may transfer out of the diluting compartment during electrodialysis of whey. A high lactose content in the brine stream may create waste disposal problems related to the biochemical oxygen demand of lactose.
A need exists, therefore, for additional methods and membranes useful in electrodialytic purification of whey and other liquids.